Users of the current PS1 DR1 data should be aware of a few issues and inconsistencies in the data.
Missing data
A small number (about 0.01%) of catalog objects are missing. The missing objects are near declination = +30 degrees. Currently we do not expect these objects to be filled in before the DR2 data release (which will include all the multi-epoch PS1 photometry).
The original 2016 December 19 release of database was missing much more data over about 1.5% of the sky. The missing regions fell at the boundaries between the declination strips that are used to organize the underlying databases. The figure below shows a schematic representation of the locations where data are missing. A database update on 2017 February 2 filled in almost all of the missing objects.
For users who have done large-scale queries and want to get just the new objects, there are 3 new tables to make this easier. ObjectThinMissing has 132 million objects that were previously missing from ObjectThin; MeanObjectMissing has the same 132 million objects that were missing from MeanObject; and StackObjectThinMissing has 40 million objects that were missing from StackObjectThin. It should be possible to run most queries again using these much smaller tables to quickly fill in missing data.
Astrometry differences between stack and mean positions
The astrometry for the mean object positions in PS1 (the raMean and decMean columns in the ObjectThin table) were updated shortly before the PS1 DR1 release using Gaia as a reference catalog. The previous version of the astrometry used 2MASS as a reference catalog. That version had systematic absolute errors of up to ~0.1 arcsec depending on the location in the sky. The new Gaia-based astrometry is much more accurate, with median errors of only about 5 milliarcsec. See the PS1 Astrometry page for more details.
However, the astrometry in this update was only applied to objects that are detected in the single-epoch warp images. Objects too faint to be detected in single epoch images are detected only in the deeper stack images. The RA and Dec measurements for stack objects have not been placed on the Gaia frame and so have larger systematic errors than mean measurements.
Stack-only objects are within ~1.3 mag of the 5σ detection limit of the survey and so tend to have noisier position measurements. That means that the position for an object is not greatly degraded by the systematic error. But if the positions of multiple faint stack objects are averaged (e.g., by using a set of stack objects as an astrometric reference), the systematic errors in the astrometry may become noticeable.
The bottom line is that users should take care with the astrometry of objects that are not detected in the mean filter magnitudes. We expect this to be fixed in the DR2 release.
Astrometric and photometric differences between images and catalogs
The astrometric world coordinate system keywords in the FITS image headers were derived at the time the image pixels were processed. The catalogs have had significant post-image processing to improve both the astrometric and photometric calibrations of the catalog objects. The Gaia DR1 catalog, which was released well after the pixel processing was complete, was used to improve the astrometric calibration of the catalog object positions. The ubercal procedure was used to modify the photometric calibration using sources observed in multiple images. That allowed correcting small errors in the photometry that depended on the exposure conditions, the epoch of the observation, the position in the detector, etc.
Since both of these calibrations were completed after the creation of the images, the image FITS headers do not reflect the improvements. Measurements made directly from the images will consequently not be as accurate either photometrically or astrometrically as measurements in the catalog.
We may at some future point update the astrometry in the images to correct for larger shifts between the Gaia-based coordinates and the older 2MASS-based coordinates. Accurate corrections of the smaller scale astrometric and photometric variations from the catalog-phase processing would require reprocessing the images. That is not being contemplated at this time.
FITS image format quirks
The FITS images have several unusual characteristics. They are described in more detail in the image cutout documentation. We may fix this at some time in the future, but since we have 2 petabytes of PS1 FITS images, this is unlikely to be done soon.
The images are compressed using the FITS tile-compressed image convention. The format can be read by most FITS libraries or can be converted back to a standard uncompressed FITS image using the fpack utility. Note that this applies only to the full skycell FITS images – FITS cutout images are not compressed.
One quirk of these images is that they use the obsolete WCS keywords PC001001, PC001002, PC002001, PC002002 instead of the FITS standard keywords CD1_1, CD1_2, etc. Many software packages automatically handle the old PC keywords, but some require special processing (e.g., in IDL you should call the fits_cd_fix procedure to modify the header). If you do not use these keywords, you will find that the RA pixel spacing has the wrong sign (it is positive instead of negative).
The image pixel values have been non-linearly scaled using an arc hyperbolic sine (asinh) transformation that converts them to a pseudo-magnitude scale related to the asinh magnitudes (aka "luptitudes") that are used in the Sloan Digital Sky Survey. The scaling is determined by the BSOFTEN/BOFFSET keywords in the FITS header. Here are the relevant lines from the header of the skycell 1725.051 g-band image:
BZERO = 3.283630371094E+00 / Scaling: TRUE = BZERO + BSCALE * DISK
BSCALE = 2.008622776974E-04 / Scaling: TRUE = BZERO + BSCALE * DISK
BSOFTEN = 8.739592975627E+01 / Scaling: LINEAR = 2 * BSOFTEN * sinh(TRUE/a)
BOFFSET = 2.654963016510E+00 / Scaling: UNCOMP = BOFFSET + LINEAR
The BZERO and BSCALE keywords are the standard FITS keywords for converting an integer image to a floating point value. If v is the original float pixel value (after applying BZERO/BSCALE to the integer value), these equations convert v to a standard linear flux:
x = v * 0.4 * ln(10) = v / 1.0857362
flux = boffset + bsoften * 2 * sinh(x)
= boffset + bsoften * (exp(x) - exp(-x))
= boffset + bsoften * (10**(0.4*v) - 10**(-0.4*v))
The 3 equations for the flux are equivalent. The third equation shows the connection of the values to asinh magnitudes.
The asinh flux scaling also applies only to the full skycell FITS images – FITS cutout images are converted to standard linear fluxes.
